Solid-State NMR and Modeling in Materials Science

Collaborations

“Synthesis and Processing of Active Calcium Phosphate Cements”, funded by the Swedish Foundation for Strategic Research (2016-2021). Håkan Engqvist (Uppsala Univ, PI); Molly Stevens (Karolinska Institute; Imperial College London); Peter Thompsen (Gothenburg Univ)

A selection from Stockholm University publication database

• Exotic structural motifs in aluminosilicate glasses quantified by solid-state NMR and molecular dynamics simulations

  2021. Baltzar Stevensson, Mattias Edén. Journal of Non-Crystalline Solids 569

  Article

  Conventional structural models of aluminosilicate (AS) glasses assume that there are no direct bonds between four-coordinated Al (Al^[4]) species and non-bridging oxygen (NBO) anions, along with the (near) absence of Al^[4]–O–Al^[4] linkages (the “Loewenstein Al avoidance rule”). Yet, accumulating evidence from advanced solid-state NMR experiments on AS glasses that incorporate (moderately) high field-strength cations, notably so those of rare-earth metals (RE³⁺), reveal significant Al^[4]–NBO and Al^[4]–O–Al^[4] populations. Here we review such NMR experimentation that enable a direct probing of these “exotic” structural motifs. We also re-analyze and discuss previously presented experimental and modeling results of a large set of RE₂O₃–Al₂O₃–SiO₂ glasses with RE={La, Y, Lu, Sc}, where the structure/composition-related parameters that dictate the degrees of Al^[4]–NBO and Al^[4]–O–Al^[4] bonding in AS glasses are identified, leading to new quantitative composition–structure relationships. Moreover, for a given AS composition, criteria are presented for whenever Al^[4]–O–Al^[4] linkages must exit in its glass structure.

  Read more about Exotic structural motifs in aluminosilicate glasses quantified by solid-state NMR and molecular dynamics simulations

• Refined Structures of O-Phospho-l-serine and Its Calcium Salt by New Multinuclear Solid-State NMR Crystallography Methods

  2021. Renny Mathew, Baltzar Stevensson, Mattias Edén. Journal of Physical Chemistry B 125 (39), 10985-11004

  Article

  O-phospho-l-serine (Pser) and its Ca salt, Ca[O-phospho-l-serine]·H2O (CaPser), play important roles for bone mineralization and were recently also proposed to account for the markedly improved bone-adhesive properties of Pser-doped calcium phosphate-based cements for biomedical implants. However, the hitherto few proposed structural models of Pser and CaPser were obtained by X-ray diffraction, thereby leaving the proton positions poorly defined. Herein, we refine the Pser and CaPser structures by density functional theory (DFT) calculations and contrast them with direct interatomic-distance constraints from two-dimensional (2D) nuclear magnetic resonance (NMR) correlation experimentation at fast magic-angle spinning (MAS), encompassing double-quantum–single-quantum (2Q–1Q) 1H NMR along with heteronuclear 13C{1H} and 31P{1H} correlation NMR experiments. The Pser and CaPser structures before and after refinements by DFT were validated against sets of NMR-derived effective 1H–1H, 1H–31P, and 1H–13C distances, which confirmed the improved accuracy of the refined structures. Each distance set was derived from one sole 2D NMR experiment applied to a powder without isotopic enrichment. The distances were extracted without invoking numerical spin-dynamics simulations or approximate phenomenological models. We highlight the advantages and limitations of the new distance-extraction procedure. Isotropic 1H, 13C, and 31P chemical shifts obtained by DFT calculations using the gauge including projector augmented wave (GIPAW) method agreed very well with the experimental results. We discuss the isotropic and anisotropic 13C and 31P chemical-shift parameters in relation to the previous literature, where most data on CaPser are reported herein for the first time.

  Read more about Refined Structures of O-Phospho-l-serine and Its Calcium Salt by New Multinuclear Solid-State NMR Crystallography Methods

• The Carbonate and Sodium Environments in Precipitated and Biomimetic Calcium Hydroxy-Carbonate Apatite Contrasted with Bone Mineral

  2021. Ozlen F. Yasar (et al.). The Journal of Physical Chemistry C 125 (19), 10572-10592

  Article

  Bone mineral consists of calcium hydroxy-carbonate apatite (HCA) that incorporates other minor cation substituents, primarily Na+ (0.5-0.8 wt %). We examine the carbonate species in various HCA specimens with variable CO32- contents (4-10 wt %), encompassing phases prepared by precipitation and a biomimetic specimen formed from a bioactive glass inside a simulated body fluid as well as bone tissue from beagle dog. Using magic-angle spinning (MAS) nuclear magnetic resonance (NMR) along with infrared spectroscopy experiments, we identified and quantified carbonate anions replacing either hydroxyl (A-type CO32-) or phosphate (B-type CO32-) anions in the HCA lattice, along with the carbonate species present in the amorphous surface layer present at all synthetic and biogenic nanocrystalline HCA particles. Advanced C-13-based NMR experimentation enabled the selective detection of the minor (CO32-)-C-13 population of intact bone monoliths, whose C-13 NMR signals are otherwise swamped by those from collagen, unless chemically invasive deproteination procedures are invoked. The CO32- species present in 4 week- and 8 month-old bone of the alveolar process from beagle dog revealed mainly B-type lattice sites and carbonates present in the amorphous surface layer. No tissue aging effects were observed in the local CO32- environments. Likewise, NMR revealed very similar Na-23(+) parameters in HCA, regardless of its synthetic or biogenic origin or degree of structural order. A combination of interatomic-distance-sensitive MAS NMR experiments allowed the identification of the local environments of the various carbonate, phosphate, hydroxyl, and water species present in both the interior and the surface layer of the synthetic HCA particles. We highlight the similarities/differences in the chemical speciation and the spatial distribution of CO32- anions present in carbonate-bearing amorphous calcium phosphate (ACP) relative to the ACP-like surface layer of HCA and discuss the C-13 NMR peak assignments of the up to four coexisting CO32- populations in the synthetic/biogenic HCA phases.

  Read more about The Carbonate and Sodium Environments in Precipitated and Biomimetic Calcium Hydroxy-Carbonate Apatite Contrasted with Bone Mineral

• Structural Role and Spatial Distribution of Carbonate Ions in Amorphous Calcium Phosphate

  2021. Ozlen F. Yasar (et al.). The Journal of Physical Chemistry C 125 (8), 4675-4693

  Article

  The local structures of a series of amorphous calcium phosphate (ACP) phases with increasing carbonate contents (2-14 wt %) were studied by multinuclear H-1, C-13, Na-23, and P-31 magic-angle spinning (MAS) nuclear magnetic resonance (NMR) experiments together with infrared (IR) spectroscopy. A model for carbonate incorporation into ACP is proposed, where carbonates enter as CO32- anions, whose equal C-13 chemical shifts (delta(C) = 168.6 ppm) imply identical local CO(3)(2- )environments in the ACP structure, irrespective of its carbonate content. The bicarbonate contents were negligible, except in the CO32--richest ACP sample, where HCO3- ions accounted for 4.3% of all carbonate species. The HCO3- anions in ACP are characterized by C-13 and H-1 chemical shifts delta(C) = 162 ppm and delta(H) = 14 ppm, respectively, as deduced from C-13{H-1} heteronudear correlation (HETCOR) two-dimensional (2D) NMR experiments. Regardless of the precise carbonate content, the ACP samples contained very similar amounts of water (approximate to 15 wt %)- most of which is structure-bound (approximate to 70%) and the remaining physisorbed-along with acidic protons of HPO42- anions, which typically accounted for approximate to 20% of the phosphate speciation. The local proton and phosphate environments were probed further by heteronuclear H-1/P-31 2D NMR experiments. We also extracted the Na-23 NMR parameters of the Na+ sites present in minute amounts (0.1-1.1 wt %) in the ACP specimens, which along with their C-13/P-31/H-1 NMR counterparts of the CO32-, HCO3-, PO43-, and HPO42- moieties are discussed and contrasted with previous reports on Na/carbonate-bearing Ca phosphate phases, such as synthetic and biogenic hydroxy-carbonate apatite. The spatial distribution of the carbonate species was determined from advanced homonuclear C-13 and P-31 double-quantum together with heteronudear C-13{P-31} MAS NMR experimentation, where each technique provided independent and consistent evidence for randomly distributed CO32- moieties.

  Read more about Structural Role and Spatial Distribution of Carbonate Ions in Amorphous Calcium Phosphate

• Metadynamics Simulations of the pH-Dependent Adsorption of Phosphoserine and Citrate on Disordered Apatite Surfaces

  2021. Baltzar Stevensson, Mattias Edén. The Journal of Physical Chemistry C 125 (22), 11987-12003

  Article

  Both citrate ions and proteins with phosphorylated side chains are believed to play central roles in bone mineralization. Using metadynamics simulations, we elucidated the pH-dependent surface binding of citrate and O-phospho-L-serine (Pser) at structurally disordered (100) and (001) surfaces of Ca hydroxyapatite (HA), the mother structure of bone mineral. The binding strength of Pser at the (100) surface increased concurrently with the pH value from 4.5 to 14.0 and remained consistently stronger than that at the (001) surface. In contrast, citrate revealed very similar adsorption affinities at both (100) and (001) surfaces throughout 7.4 <= pH <= 12.3, whereas adsorption at the (100) surface was favored at the lowest (4.5) and highest (14.0) pH values. The two most stable/probable binding modes of citrate involved either a simultaneous anchoring of all three COO- groups such that the molecule caps the HA surface or a tilted configuration stemming from the dual binding of the central and one terminal COO- moiety. The surface adsorption of Pser is dominated by its phosphate group, which participates in all significant binding modes, with the two most prominent ones featuring either a co-binding of the PO4 and COO- sites or a linear alignment of the molecule with the surface by the simultaneous anchoring of all three phosphate, carboxy, and amino groups. The latter constitutes the most stable binding mode at the (100) surface for pH = 7.4. We also introduced a straightforward analysis protocol based on Debye-Hiickel energies, enabling the quantification of the relative contributions of each functional group of an adsorbed molecule, as well as its underlying interaction energies with the surface. Both the Pser and citrate adsorption occurred predominantly through electrostatic Ca2+-COO-/PO42- interactions, along with overall minor H-bond contributions, except for the Pser binding at the PO4-richer (001) HA surface for pH values between 7.4 and 12.3. We highlight the importance of excluding OH groups in the HA surface model to better mimic real nanocrystalline apatite particles and improve the accuracy of the adsorption modeling.

  Read more about Metadynamics Simulations of the pH-Dependent Adsorption of Phosphoserine and Citrate on Disordered Apatite Surfaces

Show all publications by Baltzar Stevensson at Stockholm University